This invention relates to sideslip testers for vehicles, and, in particular, to sideslip testers for multi-axle vehicles which can measure the scrub angle of the vehicles' tandem axles.
As a vehicle rolls down the road, there should not be any lateral forces on the tires. When lateral forces are present, the net effect of these forces is the same as if the wheels were being dragged sideways as they roll down the road. These lateral forces will scuff the tires, causing them to wear prematurely.
Sideslip is a measure of the magnitude of these lateral forces and is measured in units of feet/mile or meters/kilometer. For example, a sideslip measurement of twenty feet/mile means that if that vehicle were driven in a straight line for one mile, the tires would have been scuffed sideways a distance of twenty feet.
The largest influence on the sideslip measurement is total toe. A positive sideslip measurement generally reflects a toe-in condition and a negative sideslip measurement generally reflects a toe-out condition. A sideslip tester can thus be used as a diagnostic tool to determine if a particular vehicle has alignment problems which should be checked further.
Sideslip meters are known, and in our U.S. Pat. No. 5,129,149, which is incorporated herein by reference, we described a sideslip meter which may be used with automobiles. The sideslip meter disclosed therein measured the sideslip of the wheels on the front and back axles of the car independently of each other. Although that meter could be used for multi-axle vehicles to measure the sideslip of the tires of each set of wheels independently, multi-axle vehicles, having pairs of tandem axles, have particular problems which that meter does not address.
In multi-axle vehicles with tandem axles, the sideslip of a particular axle is not due solely to the toe condition of the wheels of that axle. Rather, it is affected by the geometric relationship of the tandem axles. The tandem axles should be parallel to each other. If not, a scrub angle exists, which, like sideslip, is measured in feet/mile or meters/kilometer. Because the wheels of the tandem axle are close together (averaging only 52 inches from center to center), if a scrub angle exists, the forward motion of the wheels of one of the axles of the tandem axle pair will create a sideslip in the tires of the other axles. Further, the steering axle must be operated to overcome the lateral forces exerted thereon as a result of the existence of the scrub angle in the tandem axles. This creates a sideslip on the steering axle that is not due solely to the toe condition of the wheels of the steering axle.
The presence of a scrub angle in the tandem axle pair thus affects the sideslip for all the wheels of multi-axle vehicles. Therefore, to accurately determine sideslip on a multi-axle vehicle, the sideslip tester or meter must be able to determine the scrub angle of tandem axles. To do this, the sideslip meter must be able to recognize that the tires of tandem axles are rolling over it. Currently available sideslip testers or meters, however, are not believed to be able to compute sideslip measurements for more than two axles, nor are they believed to be able to determine scrub angles for tandem axles.